1. Field of the Invention
The present invention relates to a plasma processing method and plasma generator for processing semiconductor devices with a plasma during processes for fabricating the devices.
2. Description of the Related Art
In processes for fabricating semiconductor devices, various kinds of processes are performed, using a plasma generator such as a plasma doping system, a plasma-enhanced CVD system, a plasma etching system, and a plasma ashing system. In any of these kinds of processes, a plasma is generated, and processing is performed, using the generated plasma.
In a plasma doping system, a glow discharge plasma is generated, for example, inside a gaseous ambient containing impurity atoms to be doped, to implant the impurity atoms into a sample.
In a plasma-enhanced CVD system, an r.f. glow discharge plasma is generated, for example, in a reactive gas ambient. The reactive gas species is decomposed, thus obtaining atoms. These atoms are deposited as a thin film.
In a plasma etching system, a plasma is generated, for example, in a reactive gas ambient. The etching process is made to proceed by the action of neutral active species. This system includes a plasma ashing system. Alternatively, the etching process is made to proceed by the combined effect of the neutral active species and reactive gas ions. Or, a diverging magnetic field is formed by a magnetic field, and a plasma is radiated from an ion source along the diverging field. As a result, a sample placed in the plasma flow is etched.
In the above-described plasma generator, when a plasma is generated, ultraviolet radiation is emitted from the region where the plasma is created. Therefore, the ultraviolet radiation impinges on the sample under plasma processing. Consequently, the sample is damaged.
One example of this is described by taking hydrogenation using plasma doping as an example. Plasma hydrogenation is a technique consisting of implanting hydrogen into a poly-crystalline silicon film to remove crystal grain boundaries from the silicon film. In this processing, energetic ultraviolet radiation having wavelengths ranging from 110 nm to 180 nm is emitted from the plasma generation region. When a poly-crystalline silicon device is exposed to such UV radiation, the device is readily damaged.
As an example of characteristic of plasma hydrogenated TFT (thin-film transistor), drain current-gate voltage curve (Id-Vg curve) is shown in FIG. 7. In this graph, the drain current is plotted on the vertical axis, whereas the drain voltage is on the horizontal axis.
The device whose characteristic is indicated by the solid-line curve (a) of FIG. 7 was obtained by hydrogenation. This characteristic is considerably better than the characteristic of a device undergone no hydrogenation. However, the characteristic is not very excellent because of damage caused by ultraviolet radiation produced by a glow discharge and because of damage sustained by energetic particles.
Then, the surface of the device to be processed was covered with quartz glass obstructing hydrogen flux. Thereafter, the surface was exposed to ultraviolet radiation emitted from the plasma generation region at 285.degree. C. As a result, a characteristic curve indicated by the broken line of (b) in FIG. 7 was derived. As can be seen from this characteristic curve, the device characteristic was greatly deteriorated by the exposure to the ultraviolet radiation.
The device was then annealed at 285.degree. C. for 1 hour. We checked if this annealing could repair the damage caused by the UV irradiation. As indicated by the characteristic curve (c) indicated by the dot-and-dash line, the device characteristic was healed to some extent, not completely. Accordingly, when the device was exposed to UV radiation, it was damaged, so that the characteristics were deteriorated.